Major Instream Works on the Mulkear River Underway

Since the 1850s there have been three major drainage schemes on the Mulkear River, the latest of which was completed in 1998. In addition, there has been continuous drainage maintenance of the river for flood relief, and this maintenance continues today. The earlier drainage schemes reduced the base level of the river, by up to 4m below the natural river bed levels along the main channel, removed large rock, overhanging vegetation and woody substrate. Coarse cobble and gravel was also removed. This type of substrate is still absent in many parts of the main channel of the Mulkear River.

A consequence of this drainage work has been the loss of channel length, through the removal of meanders (bends in the river) and the loss in instream habitat through the removal of instream substrate. These practices have given a more uniform channel and in certain stretches of the river, the altered channel has led to significant bank erosion. This in turn can create a major problem with eroded river bank silt being deposited into the channel potentially impacting on salmon spawning beds. The cumulative effect of these measures has straightened the river and greatly reduced the complexity of the habitat which had lead to a reduction in fish numbers.

MulkearLIFE, with our project partner Limerick County Council, is delighted to now report the start of major instream works on the Mulkear River to address some of these concerns. The works will not impact negatively or restrict the flow of water or create a risk of flooding rather they will greatly enhance the complexity of the river and river bank biodiversity.

Over one year of planning has gone into the development of a habitat restoration plan for the Mulkear River, the implementation of which is now underway. The initial planning included comprehensive walkover and boat surveys to assess the habitat of the Mulkear River. The survey work was designed to make recommendations on instream restoration measures which could be carried in line with the work programme and objectives of the EU funded MulkearLIFE project.

Completed Rubble Mat (Image: Ruairí Ó Conchúir)

The work, which is being carried out by a large team of skilled and committed Limerick County Council workers, commenced in early July 2011 with the placement of the first rubble mats on the main Mulkear channel. This took place down stream from Dromkeen Bridge. The main objective of this work is to restore degraded habitats along stretches of the river. The need for such instream measures arises from the arterial drainage work carried out on the Mulkear which altered the pre-existing natural riffles.

Installation of Rubble Mat (Image: Ruairí Ó Conchúir)

Rubble mats essentially perform the same function by mimicking the natural riffle habitat which were present pre-drainage. The rubble mat reduces the cross-sectional area of the river thereby increasing flow velocities at low summer flows. The faster flowing area on top of the rubble mat is quickly colonised by a range of aquatic vegetation. In addition, a considerable variety of invertebrates favour such conditions and colonise the rubble mat in significant numbers. This level of colonisation happens within months and is indeed already taking place. The fast flowing water also provides exceptional habitat for young salmon and trout and with invertebrate colonisation having taken place will provide them with an adequate food supply.

When the rubble mat is dished towards the centre of the channel the velocity through the dished section may be adequate to maintain scour for an excavated pool downstream. Such pools will also provide resting areas for adult salmon. To date six rubble mats have been installed downstream of Dromkeen Bridge, the largest of which requiring in excess of 100 tonne of rock. Additional rubble mats will be installed upstream and downstream of Brittas Bridge.

Long-reach excavator from the river (Image: Glen Wightman)

MulkearLIFE and Limerick County Council have prioritised the implementation of these instream measures as part of the overall work programme of MulkearLIFE in the period 2011 to 2014. The overall objective of the habitat restoration plan is to enhance habitat complexity and quality in the wider river ecosystem.

Completed Rubble Mat (Image: Ruairí Ó Conchúir)

The work will be beneficial to a range of species associated with the Lower Shannon SAC. These species include the target species of MulkearLIFE namely, Atlantic Salmon, Sea Lamprey and European Otter. The work will also benefit other lamprey species (river and brook), brown trout, white-clawed crayfish (for which traps were set pre construction and removed post construction), kingfisher, heron, dipper and various aquatic invertebrates. All proposed structures are designed to minimise conveyance of flood water and are being built during periods of low water flow.

The staff of MulkearLIFE, Inland Fisheries Ireland and Limerick County Council are working in partnership to complete the work this summer with the full co-operation and support of local NPWS conservation rangers. The construction of the rubble mats will prioritized where they have the greatest potential beneficial impact on Atlantic Salmon.